The present invention relates to a method of feedback-controlling the air-fuel ratio of an internal combustion engine, and more particularly, it relates to a method of this kind wherein the air-fuel mixture supplied to the engine is feedback-controlled in response to the output of an exhaust gas ingredient concentration sensor having output characteristics in approximate proportion to the exhaust gas ingredient concentration.
Among methods of feedback-controlling the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine (referred to hereinafter as "supply air-fuel ratio") in response to the output of an exhaust gas ingredient concentration sensor having output characteristics proportional to the exhaust gas ingredient concentration, there is one proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 59-208141, in which a desired air-fuel ratio coefficient representing a desired air-fuel ratio of the feedback control is set in response to operating conditions of the engine, and an air-fuel ratio correction coefficient is calculated according to the set desired air-fuel ratio coefficient and output from the exhaust gas ingredient concentration sensor, in order to determine an amount of fuel to be supplied to the engine by multiplying a basic value of the amount of fuel to be supplied to the engine by these coefficients.
In general, when fuel is actually supplied to the engine, it has cooling effects, so that the charging efficiency is enhanced to thereby increase the weight of intake air. The above proposed method does not take the cooling effects of actually-supplied fuel into consideration, and therefore it suffers from a problem that the air-fuel ratio of a mixture drawn into a combustion chamber is deviated in a leaning direction from a desired value due to the cooling effects of the supplied fuel, which makes it impossible to obtain a desired supply air-fuel ratio.